The present invention relates to a positioning device for an exposing and developing apparatus for processing printing forms which are transported onto an exposure table by transport rollers, and more particularly to a device for positioning the individual printing forms both longitudinally and transversely.
The production of printing forms carrying lettering and images, for example, printing plates which, after exposure and development, are immediately suitable for printing, is carried out by first charging the printing plates electrostatically and then projecting an original, as an image, onto the individual printing plates. Following termination of this exposure process, each printing plate is developed with a developer, fixed and decoated, after which the plate is ready for the printing operation.
German Offenlegungsschrift No. 30 12 761 (equivalent to U.S. Pat. No. 4,334,472) describes a mechanism for feeding and transporting printing plates to an exposure table by means of transport rollers. The uppermost of the printing plates, which are stacked in a plate holder, is gripped by a swiveling lifting cylinder, swung through a semi-circular arc movement, and deposited on a suction plate of the exposure table. The transport rollers in each row are arranged on either side of the suction plate and are driven by a drive roller via drive belts. Those transport rollers which are the first in each row (seen in the direction of printing plate travel) are arranged on swivel blocks which are swiveled outwardly from the rows before the printing plate is put down. This enables the forward part of the printing plate to be deposited on the suction plate without jamming. As soon as the printing plate rests on the suction plate, the first transport rollers are swiveled towards the lateral edges of the printing plate which is then transported forward, its lateral edges being guided in grooves provided in the transport rollers. The printing plate is moved forward on the suction plate until it contacts a stop pin. As soon as the printing plate is positioned by the stop pin, reduced pressure is applied to the suction plate. The printing plate is thus retained in its exposure position. Thereafter, the printing plate is charged by a corona device and subsequently exposed. After exposure, the stop pin is lowered, so that the printing plate can be transported onto a developing table by the transport rollers. In this device, the printing plate is only positioned in the direction of transport, i.e. longitudinally, but not transversely.
German Patent Application No. 31 22 321. 4 (equivalent to U.S. Ser. No. 382,288, U.S. Pat. No. 4,462,678) describes a device for feeding and transporting printing plates onto an exposure table with the aid of rollers which are arranged in pairs and are fastened by means of blocks on two roller-rails. The roller rails are located at both sides of the exposure table, in the direction of transport. The transport rollers which are positioned first, seen in the transport direction of the printing plate, are pivoted outwardly before the printing plate is deposited and, after the printing plate has been deposited, are pivoted in again in order to take over the roller transport, together with the remaining rollers. The printing plate is transported forward on the suction table until it contacts a stop which can be lowered into the suction plate.
When the printing plate has been positioned by the stop, reduced pressure is applied to it so that it is retained in its exposure position. In this device, too, positioning by means of a stop takes place in the direction of transport of the printing plate only, i.e. longitudinally, but not transversely. In the exposure position, the roller rails are swung entirely away from the exposure table, parallel to themselves, thus enabling the outer edges of the printing plate to be exposed without hindrance.
In the known devices, positioning of the printing form is performed in the longitudinal direction only, by bringing the forward edge of the printing form into contact with a stop, without also aligning the back edge of the printing form or, respectively, exerting a contact pressure in the stop direction. No lateral positioning or alignment takes place, apart from the fact that the two longitudinal edges of the printing form are guided in the grooves provided in the transport rollers. For this reason, a precise and reproducible alignment of the printing form on the exposure table is possible to a limited extent only with the known devices.